Micro-Controller Unit Application in Fuzzy Battery Equalization Control for Battery String

Transcription

1 i icro-controer Unit Appication in Fuzzy Battery Equaization Contro for Battery tring Yuang hung ee, ember, EEE, zu Han Chen, and Yi Pin Ko Abstract An inteigent battery equaization scheme is presented, based on and accordingy designed as fuzzy ogic controed battery equaization controer (FC-BEC). t can effectivey reduce equaization time. The proposed individua ce equaization (CE) scheme is a bidirectiona dc-dc converter, based on the Cûk converter operated at discontinuous inductor current mode (DC). A state space approach is adopted to inspect the stabiity of the proposed FC-BEC. atab/imuink simuation and experimenta resuts can be used to verify the stabiity of FC-BEC. n this artice, an 8-bit micro-controer unit (CU) is used to accompish the FC-BEC, which can reduce voume and compexity of the hardware used for the FC-BEC. B. NTRODUCTON ecause the inherent votage of a singe ithium-ion battery ce is very ow, in many practica appications such as eectric vehices, notebooks and genera consumer eectronic products, severa ces are cascaded in a series string in order to improve votage and capacity. mbaanced ce votage within a series string can be attributed to the differences in the ces interna resistance. iterature references present battery management systems (B), which can not ony monitor the battery votages and equaization currents but can aso prevent imbaances during charging and discharging in series connected battery ces []-[3]. ndividua ce equaization (CE) schemes have been proposed to equaize battery strings. The bidirectiona battery equaization scheme has many advantages such as higher equaization efficiency and a moduar design approach. The disadvantage of this equaization scheme is that the stored energy in the inductor is transferred ony to the weaker ce in the (-D)T duty cyce. The equaization time of this equaization scheme is therefore poor. An overcharge wi vaporize the active materias in the battery, increase the interna pressure and produce a higher risk for exposion. Over-discharge wi dissove the copper in the eectroyte and form copper dendrites that harm the battery and shorten the battery ce ife. An CE must be used to diagnose and correct This work was supported by the Nationa cience Counci of Taiwan, R.O.C., under Grants NC E-3--URD. Y.-. ee is with the Department of Eectronic Engineering, Fu-Jen Cathoic University, Taipei 45, Taiwan, R.O.C. (e-mai: ee@ee.fu.edu.tw)..-h. Chen is with the Department of Eectronic Engineering Fu-Jen Cathoic University, Taipei 45, Taiwan, R.O.C. (e-mai: a9356@st.fu.edu.tw). Y.-P. Ko Graduate nstitute of A..E Fu-Jen Cathoic University, Taipei 45, Taiwan (e-mai: koaiya@yahoo.com.tw). any ce votage imbaances in a ithium-ion battery string [4], [5]. The fuzzy ogic contro (FC) method is suitabe for predicting the noninear behavior of battery equaization, because it has more adaptabiity, robustness and does not require an accuratey mathematica mode. The proposed fuzzy ogic controed battery equaization controer (FC-BEC) can reduce the equaization time and maintain safe operation for each ithium-ion ce in the battery string during the charge and discharge state [6]. tabiity anaysis, one of the most important subects for the controed system, has to determine whether during norma running the controed system is stabe or not. The state space approach is a direct and powerfu anaytica method for the stabiity anaysis of a FC [7]. By probing into the reation between the reative infuence of each rue of the rue base and the inguistic traectory of the dynamic system, it can be used for proving whether a FC reaches stabiity. o the state space approach is adopted to inspect the stabiity of the proposed FC-BEC. n accordance with consumers ardent demands for a onger battery ife cyce, an 8-bit micro-controer unit (CU) is empoyed to impement the battery management system (B) with the proposed battery equaization scheme. Compared with the conventiona B with the battery equaization controer made up with 85, A/D converter and severa ogic Cs, it not ony can reduce the numbers of C devices for obtained price advantage, but can aso save space and overa power consumption. n addition, it can increase the capacity of charging and discharging the series connected battery ces, and as a resut has the individua ces reach better equaization performance.. ODENG OF CE The studied battery charging system with the proposed CEs and the microprocessor based B is shown in Fig.. The system is composed of N battery ces, (N-) CEs, (N-) inductors and OFET switches. The proposed CE is a bidirectiona dc-dc converter based on the Cûk converter shown in Fig.. t is comprised of two inductors and, an energy transfer capacitor C, and two power OFETs with body diodes as the battery ce-baancing switches. The ce votage baancing contro agorithm for this equaization scheme is instructed by a microprocessor based B. The energy between the adoining battery ces is transferred through the energy transferring capacitor. The initia capacitor votage C equas B B. For exampe, the PW contro signa turns on/off the Q to transfer some of the stronger ce

2 C harging source s Q B C E K C E K D D D B C Q Q C B Q Q 3 B D D 3 3 s N Output tage and Driving igna Generator BN N D Q N Output tage and Driving igna Generator ensor of Battery tates Fuzzy ogic Equaization C ontroer icro-controer Unit Based Battery anagem ent ystem Fig.. The configuration of the studied battery charging system with CE and B votage, B, to a weaker ce, B. The stronger ce energy is transferred from ce B to ce B. Conversey, if ce B is stronger than the weaker ce B, the energy is transferred from ce B to ce B by controing the Q. The proposed battery equaization scheme was designed to operate at DC to obtain zero current switching (ZC) in OFET switches [8], [9]. t is the same as the continuous inductor current mode (CC), but with the different operated frequency. The proposed method can be used to reduce the switching osses in OFET switches and increase the equaization efficiency during the ce votage baancing processing []. s B Q D C C Q D Fig.. The proposed CE Using the votage baance across and over one time period in the boundary condition between CC and DC, the votage and current transfer ratio of the th CE can be obtained as: B D () D B where D is the duty ratio of the converter switches. From e = di/, the peak inductor currents are: B ( DT ) i peak () B s i peak B ( D) T (3) Because the average equaization current is: a i a (4) peak the boundary conditions for operating in DC can be expressed using the foowing two equations: T (5) ( D) B D T (6) ( D) B where T s = /f and f is the switching frequency of the converter. Equations (5) and (6) are the boundary conditions for the proposed battery equaization scheme to differentiate between CC and DC. The equivaent circuit of DC for B > B is shown in Fig. 3 and anayzed as foows: ) tage, [Fig. 3, t <t<t ]: The OFET Q is turned on and the diode D is turn off. The switching variabe is u = and u* =. The energy stored in the capacitor is charged to the weaker ce B. ) tage, [Fig. 3, t <t<t ]: For this interva, OFET Q is turned off and the diode D is forced to turn on. The switching variabe is u = and u* =. The energy stored in the capacitor recharges to the weaker ce B. 3) tage 3, [Fig. 3(c), t <t<t 3 ]: The OFET Q and the diode D are both turned off, the switching variabe is u* = and u = foating state; therefore, the inductor currents are zero when C is charged to B B, and di / = di / = d C / =.

3 R B R B R B i Q i.. B i C D C C i.. B C i = i = C.. B C (c) Fig. 3. Equivaent circuit of DC for B> B Q turn-on D turn-on (c) Q and D a turn-off B B P P t t t t 3 D T T T C Fig. 4. Typica switching waveforms of CE for B > B Fig. 4 shows the typica switching waveforms of the proposed CE operated in DC for B > B. Based on the above, the dynamic system can be represented using the compact state equation: R B ( u ) B RB di i R R (7) di B u B B i C d C u u * u B ( ) * B RB R B u C C where =,, 3,, N for B > B, is composed of the charging current and the equaizing current for the other CE ce. And R B is the equivaent series resistance (ER) of the th battery ce.. DEGF FC-BEC Rue Base ( y ) ( z B B O ) Fuzzifier nference Engine Defuzzifier A( x ) d Fig. 5. Bock diagram of the FC RB RB RB B B B BEC The FC consists of the rue base, inference engine, fuzzifier and defuzzifier, as shown in Fig. 5. There are two inputs in the FC. Each input is the votage difference ( d ) between two-ces and the ce votage ( B ) in the battery string, respectivey. The fuzzy controed output BEC is the desired battery equaizing current of the proposed ce equaization scheme. Fig. 6 shows the three membership function sets for the equaizing strategy in the proposed FC-BEC that is the votage-different-sense fuzzy variabe μ A with respect to d, and the ce-votage-sense fuzzy variabe μ B with respect to B, and the output fuzzy variabe μ o with respect to the battery equaizing current BEC. Five membership functions are adopted, very arge (), arge (), medium (), sma (), and very sma () to describe the fuzzy sets. These functions are a described using the same five inguistic variabes. Degree of embership d () B ( ) BEC (A) Fig. 6. embership function of FC-BEC Based on the ce baancing contro strategies in this battery equaization technoogy, the fuzzy contro rues for the FC-BEC in the battery equaization scheme are presented as foows: is G BEC R : F is d G and d is B G THEN B BEC where G and G are the inguistic vaues for the genera signed disturbance in the th variabe for the input and output fuzzy sets. The rue base coects the contro rues that describe the knowedge and experience of the battery equaization contro in the fuzzy set. The decision rue tabe for the inguistic variabes for the FC is -dimensiona (5x5) and constructed in the inteigent contro scheme rue-based memory system, shown in Tabe. The fuzzy inference engine is an inteigent operating agorithm that transforms the fuzzy rue base into a fuzzy inguistic output. The inguistic inference resuts are converted into numerica output BEC by the defuzzifier. The fuzzy controed output BEC is the desired battery equaizing current for the proposed ce equaization scheme. The systematic design procedures for the proposed FC-BEC are summarized as foows: ) tep. Obtain the true vaue w i for the i th input membership function for x and the th input membership function for y. wi= min{ Ai ( x), B ( y)} for i=,..,5 and =,..,5 (8) ) tep.cacuate the fuzzy output vaue, μ oi (z) using w i and the k th output membership function according to each rue,

4 μ oi (z) is: oi ( z) min{ i, ( z)} for k=,..,5 (9) ok 3) tep 3. Determine the fuzzy set for output z. out ( z ) max{ o ( z ), o ( z ),, o55 ( z )} () 4) tep 4. Find the output battery equaization current BEC (A) of the FC-BEC using the inference resuts through the defuzzification process, based on the center of gravity method, expressed as: BEC m out ( z ) z m out ( z ) TABE CONTRO RUE BAE OF THE FC-BEC FOR NGUTC ARABE Ce votage, B Output 3 otage difference, d TABTY ANAY 4 () The stabiity anaysis is one of the most important subects for a controed system. The stabiity of controed systems is decided by the input or interference responses which came from the externa word. When the system is oining the imited input or interference, its output response is aso imited. After a the input or interference has disappeared, the system wi be reset to the originay static state, and the system is a stabe system [7], []. The state space approach is a direct and powerfu anaytica method for the stabiity anaysis of FC-BEC. By probing into the reation between the reative infuence of each rue of the rue base and the inguistic traectory of the dynamic system, it can be used for proving whether FC reaches stabiity. The rue-antecedent contains two process state variabes, B and d, representing the controer input variabes. The rue-consequent contains a singe contro output, BEC. The process state variabes, B and d, each have five rues separatey, so the maximum number of rues contained in the rue base is 5x5. f the crisp eement ( B, d ) hods that: p q (, ) : R (, ) R (, ) () B d p B d q B d whereμr p ( B, d ) andμr q ( B, d ) are the fuzzy reations representing the meaning of the rue-antecedent of rues p and q, respectivey. For exampe, et p p p p p p p R : if B is GB and d is Gd then BEC is GBEC (3) q q q q q q q R : if B is GB and d is Gd then BEC is GBEC ThenμR p andμr q are defined as foows: p p B, d : R p( B, d ) min G B, G d q q B, d : R q ( B, d ) min G B, G d The controed system can be represented as foows: (4) dx f ( x) bu, u ( x) (5) where f(x) is a noninear function which represents the pant dynamics, and its initia vaue, f() =. x = [i,i, C ] T, u is the scaar contro variabe that represents BEC, and Φ(x) is a noninear function representing the FC with the initia vaue Φ() =. etμ X* be the fuzzified input,μ R (x, u) be the fuzzy reation representing the meaning of the rue base, and Defuzz be any defuzzification operator, then u ( x ) Defuzz [ ( x, u )] = ax in [, ( x, u )](6) * R * X X The controed system behavior wi depend on the f(x) and Φ(x). o we can define the stabe condition of the controed system to be: dx f ( x) is stabe, u ( x) (7) n equation (7), when the traectories approach this curve, Φ(x) =, the pant f(x) wi converge to the equiibrium point. Finay we find that making an arrangement for each rue of the rue base, we can et the controed system reach the steady state very easiy. The inguistic traectory corresponds with the proposed system traectory associated with the ce votage baancing traectory in Tabe, that is: inguistic traectory = (Rue, Rue, Rue3, Rue4, Rue5, Rue6, Rue7, Rue8, Rue9, Rue, Rue, Rue, Rue3) where Rue is the fuzzy rue starting at a very arge () votage difference, a very sma () ce votage and a very arge () output. Through the contro action of the FC, the traectory reaches Rue3, which is the contro goa of the FC-BEC. This denotes that the system traectory has reached the desired goa and terminated at a very sma () votage difference, a very arge () ce votage and a very sma () output shown as in Tabe. The grey ces show the fired rue according to the specified equaization contro action of the battery string. From the observation of Tabe, the controed system with the proposed FC-BEC is stabe.. CRO-CONTROER UNT DEGN a i n n i t i a i z e H / W a r i a b e s n t e r r u p t e t t i n g T i m e r e t t i n g B a t t e r y o t a g e A t t e n u a t o r A / D e n s o r B a t t e r y o t a g e B = A D - G n d B = A D - A D B = A D - A D F B > B F B = B F B > B F B = B Y E Y E Y E Y E R C E = F u z z y T a b e a u e F r e q. C E - = o w C E = o w C E - = F u z z y T a b e a u e F r e q. C E = o w C E - = o w C E = F u z z y T a b e a u e F r e q. C E - = o w C E = o w C E - = F u z z y T a b e a u e F r e q. C E = o w C E - = o w Fig. 7. The fow chart of decided the direction of equaization

5 The battery equaization contro unit is used as the centra processing system, the 8bit CU, onix N8P7 is empoyed to reaize the structure of the battery equaization contro unit []. t is a RC-ike high performance and ow power consumption 8-bit micro-controer, which has the high EFT (Eectrica Fast Transients) protection capabiity, a 5 channe -bit ADC converter (one interna ADC channe for DD measurement). t aso incudes an interna 6Hz RC osciator for system cock. DD CE 3 CE- 4 RT 4 3 CB / Battery Hi Attenuator AD CB / Battery id Attenuator AD 5 CB / Battery ow Attenuator AD 6 AREFH / ANO 9 CE- 8 7 CE AD CE 3 s B R6 R5 C AD CE R R B. UATON AND EXPERENTA REUT A. imuation Resuts n order to vaidate the performance of the proposed EC-BEC, atab/imuink simuation is carried out for three battery ces outfitted with two CEs. The initia battery votages, uncouped inductors and energy transferring capacitors were set B = 4.(), B = 3.9(), B = 3.6(), = = 3 = 4 =μha n dc = C =47μF, r e s pe c t i v e y. The switching frequency f = 6.67kHz and duty ratio D =.5 for both B>B>B and B<B<B to ensure the proposed CE can operate in DC. The simuation resuts of the ce votage traectories under the static state, added A charging and discharging current states of the proposed FC-BEC are iustrated in the Fig.. 4 R8 CE- C R7 B with battery equaization controer, made up with 85, A/D converter and severa ogic Cs, it not ony can reduce the number of C devices, but aso can reduce the measurements more than five times. AD R3 B R4 CE Fig. 8. The proposed equaization circuit for three ces Fig. 7 shows the fow chart for deciding the direction of equaization. The first step is to initiaize the hardware parameter, set the interrupt and timer, and then utiize the attenuator to attenuate ce votage into acceptabe vaues for the ADC. The ADC reads the ce votage and transfers the anaog signa into a corresponding digita signa, and then deivers the contro signa to the equaization circuit. Repeating the above procedures, the battery equaizing process can be achieved and fufied. The proposed equaization circuit and the CU bock diagram for three ces is shown in Fig. 8. Fig. 9. Pictures of the experimenta circuits CU battery equaization unit bidirectiona Cûk converter (c) Fig. 9 shows the pictures of the experimenta circuit of the CU impemented battery equaization contro unit and the bidirectiona Cûk converter. Compared to the conventiona Fig.. imuation resuts of ce votage traectories for B>B>B static state added A charging current (c) added A discharging current

6 B. Experimenta Resuts A three-moduar ithium-ion battery stack with two of the proposed CEs was instaed for the experiment, which was used to verify the equaization performance. The experimenta battery stacks were Panasonic CGR865C ithium-ion battery ces. The battery initia votages, uncouped inductors and energy transferring capacitors were set B = 4.(), B = 3.7(), B = 3.4(), = = 3 = 4 =.3μH and C = C = 47μF, respectivey. The switching frequency and the duty ratio are f = 6.67kHz and D =.5, respectivey. The experimenta resuts of the ce votage traectories under the static state, added A charging and discharging current states of the proposed FC-BEC are iustrated in Fig.. BEC to reduce the equaization time and maintain safe operation for each ithium-ion ce in the battery string during the charge and discharge state. Because of the noninear behavior and highy changeabe characteristic of battery equaization, but with the anaysis of state space approach, we can verify the stabiity of a FC-BEC by probing into the reation between the reative infuence of each rue of the rue base and the inguistic traectory of the dynamic system. Utiizing the singe CU to accompish FC-BEC, it not ony can reduce the numbers of C devices (= price advantage), but can aso save space and overa power consumption and make the proposed individua ce equaization circuit a better commercia product. imuation and experimenta resuts verify the stabiity of the FC-BEC and guarantee the proposed FC-BEC can invariaby operate in a safe range. (c) Fig.. Experimenta resuts of ce votage traectories for B> B> B static state added A charging current (c) added A discharging current The experimenta resuts were the same as the theoretica anaysis and simuations. Therefore, the inteigent equaization method coud baance a adoining ce votages in the battery string to the same votage eve. Consequenty, each ce can be simutaneousy charged to the end-of charge votage, and the tota charging capacity of the battery string woud be increased.. CONCUON The proposed CE operated at DC can be used to achieve ZC ce-baancing contro to reduce the switching osses in OFET switches and increase the equaization efficiency during the ce votage baancing processing. After that, the FC method is used to design the proposed ACKNOWEDGENT The authors thank the Coor Fy td. for suppying the ithium-ion batteries, Panasonic CGR865C, and the Avant td. for offering the deveoping instrument of the UC. REFERENCE [] H.. enkatasety, and Y. U. Jeong, Recent Advanced in ithium-on and ithium-poymer Bateries, Proceeding of Battery Conference on Appications and Advances, the eventeenth Annua, pp , June. [] J. cdowe, A. Brenier,. Broussey, and P.avaur, ndustria ithium-on Batteries: From The aboratory to Rea Teecom Appication, Proceeding of Teecommunications Energy Conference, NTEEC 4th Annua nternationa, pp , eptember. [3] N. H. Kutkut, A oduar Nondisipative CurentDiverter for E Batery Charge Equaization, Proceeding of Appied Power Eectronics Conference and Exposition, APEC'98, Thirteenth Annua, o., pp , 998. [4] J. Chatzakis, K. Kaaitzakis, N. C. ougaris, and. N. anas, Designing A New Generaized Bateryanagement ystem, EEE Transaction on ndustria Eectronics, o. 5, No. 5, pp , 3. [5] W. F. Bentey, Ce Baancing Considerations forithium-on Battery ystems, Proceeding of the 997 Battery Conference on Appications and Advances, Twefth Annua, pp.3-6, 4-7, June 997. [6] Yuang-hung ee and Jiun-Yi Du, Design of An nteigent Controer for eries Connected ithium-on Battery trings Using FC-BEC technique, Proceedings of the st Taiwan Power Eectronics Conference TPEC, pp.7-77, eptember. [7] D. Driankov, H. Heendoom, and. Reinfrank, An introduction to fuzzy contro, (pringer), Chap. 6, pp , 996. [8] K. Nishiima, H. akamoto, and K. Harada, A PW Controed impe and High Performance Batery Baancing ystem, Power Eectronics peciaists Conference, PEC. EEE 3st Annua, o., pp. 57-5,. [9]. Broussey,. Peree, J. cdowa, G. arre, and J. artaeng, ithium on: The Next Generation of ong ife Bateries Characteristics, ife Predictions, and ntegration into Teecommunication ystem, Teecommunications Energy Conference, pp.94,. [] Yuang-hung ee and Guo-Tian Cheng, Bi-directiona Cûk Converter Appication in Ce otage Contro of Batery tring, Proceeding of the nd Taiwan Power Eectronics Conference TPEC 3, pp , eptember 3. [] hih. Chang, Automatic contro system, Chap. 3, pp.3-3,. [] 8bits icro-controer-unit, onix N8P7. Avaiabe: